Stolen from a discussion at Jerry Pournelle’s place:
Couv’s Aphorism: No country has a right to exist except that which it enforces by persuasion, diplomacy, trickery, or force of arms. Of the four, force of arms trumps all.
That is, as another commenter points out, simply existing is something a nation must actively defend. There is no inherent national “right to exist”. This means, of course, that a nation whose borders aren’t actively enforced won’t be “itself” for long.
And another “of course” (of course ;-)): undefended borders are a boon to those who want to effect the demise of a nation. Makes one wonder if applying the word “traitor” to open borders advocates isn’t merely appropriate but a bit soft.